Certain hindered amines have been found to be especially effective light stabilizers in various coatings, in diverse fibers, and in polyurethanes. One class of these materials is characterized by the 2-oxo-1,4-piperazine ring structure where one or both carbons adjacent to N-4 are tertiary carbons, as represented by structure I, (where N-4 is the NH nitrogen), ##STR1## especially where all the R groups of I are methyl. Using as exemplary those compounds of structure I where all the R groups are methyls, the most facile route to the resulting oxo-piperazine structure involves the condensation of the corresponding diamine with acetone and chloroform under strongly basic conditions, ##STR2## However, compounds corresponding to II and those with an analogous structure are themselves most readily prepared by reduction of the corresponding nitro amines--and thereon hangs our tale.
The nitro moiety in nitro aliphatic compounds is generally considerably more difficult to hydrogenate to the corresponding amino moiety than the nitro moiety in nitro aromatic compounds. Where the nitro aliphatic is an amino-substituted nitroalkane or nitrocycloalkane, problems are further compounded by the fact that the reactants are heat labile and sensitive to catalyst acidity. Where the nitro aliphatic is a nitro polyamine, especially one where the carbon bearing the nitro moiety is a tertiary carbon so as to afford a sterically hindered nitro moiety, the problems are still further compounded. In addition to the conversion and selectivity of hydrogenation being poor, none of the prior art methods is conducive to a continuous hydrogenation process. What is needed is a general method of continuously and rapidly reducing the sterically hindered nitro moiety attached to a tertiary carbon atom of aliphatic nitro polyamines at modest temperatures, say under 130.degree. C., under non-acidic conditions.
There is no prior art solution to this problem. The compromise effected by the prior art is to use Raney nickel--a spongy nickel with very high hydrogenation activity formed by treating a nickel-aluminum alloy (preferably powdered) with sodium hydroxide to dissolve the aluminum--in a batch process. It will be readily appreciated that the use of powdered Raney nickel does not lend itself to a continuous process. Where grandular alloy is used, sodium hydroxide leaching leads only to a thin, surface layer of nickel, whereas the core remains unchanged nickel-aluminum alloy. In our hands, the use of such material led to extensive decomposition of aliphatic nitro compounds during attempted hydrogenation. Although one might at first surmise that another active form of nickel on a traditional catalyst support would be an acceptable substitute for Raney nickel, this was found not to be the case even though some prior art teaches these to be acceptable where the nitro moiety is not hindered and found in e.g., a nitroparaffin. In fact, we have unexpectedly found silica to be almost uniquely effective as a support for nickel in the hydrogenation of a hindered nitro moiety in an aliphatic nitro polyamine to the amino moiety, and furthermore have found that unusually high nickel loadings are required for a satisfactory catalyst. From our novel observations it is possible to devise a method of continuous hydrogenation of a sterically hindered nitro moiety in an aliphatic nitro polyamine to an amino moiety in a high yield, with good selectivity, at moderate temperatures, and with very good overall product quality.
The patentees of U.S. Pat. No. 3,470,252 have described the continuous hydrogenation of nitroparaffins with a zirconium-modified nickel on kieselguhr, and teach that either homogeneous or supported nickel catalysts may be employed. From the broad list of cited supports (kieselguhr, silica, alumina, pumice, asbestos, carbon, and silica gel) it is clear that there was no recognition of the unique effectiveness of silica as a support, an observation which is the keystone of our invention. Swanson (U.S. Pat. No. 3,917,705) teaches a plethora of metals on a multitude of supports as possible catalysts for the reduction of nitroparaffins, from which the skilled artisan could not grasp the uniqueness of our nickel on silica catalyst. Patterson et al. (U.S. Pat. No. 3,470,250) also describe the hydrogenation of nitroparaffins citing a large number of metals, including nickel and Raney nickel, and supports as suitable catalyst combinations, from which one skilled in the art can gain no appreciation of the unique effectiveness of our high-nickel on silica catalyst system. A critical feature of their invention is a multistage reduction where further hydrogenation beyond 50% conversion is conducted in the presence of ammonia. Senkus, U.S. Pat. No. 2,393,825, teaches the reduction of nitroamines where the preferred catalyst is Raney nickel. The effectiveness of Raney nickel for the hydrogenation of the nitro moiety has often been noted, but unfortunately Raney nickel does not lend itself to use in a continuous process.